The present invention relates generally to printers, and more particularly to a method for operating a pick motor of a pick system and a separate feed motor of a feed system to pick and feed a sheet of print media for printing and to a method for operating a pick system to pick a sheet of print media for printing.
Printers include inkjet printers having a tray containing paper sheets and having a mechanism for picking the top or bottom sheet from the tray and feeding that sheet into the printing region of the printer. Some conventional inkjet printers have a pick system and a separate feed system and include a pick roller and a separate feed roller as well as a paper-sensing xe2x80x9cleverxe2x80x9d flag and a nip roller. The pick roller picks the top paper sheet from the paper tray and moves it forward along a paper path toward the feed roller. The paper sheet moves the flag just prior to entering, or as it enters, between the feed roller and the nip roller. Thereafter, the feed roller moves the top edge of the paper sheet backward along the paper path out of the grasp of the nip roller and the feed roller (while the pick roller maintains the trailing edge of the paper sheet in a fixed position) which buckles the paper sheet and aligns the top edge squarely to correct for skew. Then, the feed roller rotates forward drawing the leading edge in square, and the pick roller releases pressure on the paper sheet. Other conventional inkjet printers omit the deskew operation. What is needed is an improved method for coordinating the operation of the pick and feed systems.
Higher-cost dual channel encoders are known in printer pick and feed systems and are used to determine sheet position along both forward and reverse directions of the paper path. Lower cost single channel encoders are known in non-printing applications which can only be used to determine position only along one direction corresponding to rotation of the encoder wheel in a single direction. The encoder wheel has a circular array of transparent portions spaced apart by intervening opaque portions. The encoder has an optical sensor which changes signal level when the edges of the opaque portions rotate past the sensor. Position only along the one direction is determined by counting the number of changes in signal level. However, teetering rotational motion of the encoder wheel causes teetering changes in the signal level when an edge is being sensed by the sensor causing these signal changes to be falsely counted as motion along the forward direction leading to an erroneous determination of position. Likewise, any non-teetering rotational motion of the encoder wheel in a direction opposite to the single direction will be falsely counted as motion along the forward direction leading to an erroneous determination of position. What is needed is a method for using a printer pick system having a single channel encoder which more accurately determines position.
A first method of the invention is for operating a pick motor of a pick system and a separate feed motor of a feed system to pick and feed a sheet of print media for printing, wherein the pick and feed systems each determine sheet position, and includes steps a) through e). Step a) includes obtaining a desired pick motor velocity for the pick motor from a first function of sheet position. Step b) includes obtaining a desired feed motor velocity for the feed motor from a second function of sheet position. Step c) includes using the sheet position determined by the pick system for both steps a) and b) until the happening of a predetermined event. Step d) includes synchronizing the determined sheet position of the feed system to the determined sheet position of the pick system upon the happening of the predetermined event. Step e) includes using the sheet position determined by the feed system for both steps a) and b) after the happening of the predetermined event.
A second method of the invention is identical to the previously-described first method but also requires the pick system to be in contact with the sheet when the sheet position determined by the pick system is used for both steps a) and b) and further requires the feed system to be in contact with the sheet when the sheet position determined by the feed system is used for both steps a) and b).
A third method of the invention is identical to the previously-described first method but also includes steps f) and g). Step f) includes controlling the pick motor by comparing an actual pick motor velocity determined by the pick system with the desired pick motor velocity. Step g) includes controlling the feed motor by comparing an actual feed motor velocity determined by the feed system with the desired feed motor velocity.
A fourth method of the invention is identical to the previously-described third method but also requires the pick system to be in contact with the sheet when the sheet position determined by the pick system is used for both steps a) and b) and further requires the feed system to be in contact with the sheet when the sheet position determined by the feed system is used for both steps a) and b).
A fifth method of the invention is for operating a pick motor of a pick system to pick a sheet of print media for printing, wherein the pick system has a single-channel pick encoder including an encoder wheel and a sensor. The sensor outputs an oscillating signal having peaks and valleys when the encoder wheel is rotating. The pick system counts the number of transitions between the peaks and valleys to determine sheet position only along a forward direction of the sheet path. The fifth method includes steps a) and b). Step a) includes starting a pick operation of picking a sheet by driving the pick motor in a first direction, to move a sheet along the forward direction of the sheet path, with an input sufficient to prevent any teetering transitions which would be falsely counted as motion of the sheet along the forward direction. Step b) includes thereafter controlling the pick motor by comparing an actual pick motor velocity with a desired pick motor velocity.
A sixth method of the invention is for operating a direct current (DC) pick motor of a pick system to pick a sheet of print media for printing, wherein the pick system has a single-channel pick encoder including an encoder wheel and a sensor. The sensor outputs an oscillating signal having peaks and valleys when the encoder wheel is rotating. The pick system counts the number of transitions between the peaks and valleys to determine sheet position only along a forward direction of the sheet path. The sixth method includes steps a) and b). Step a) includes driving the pick motor with a pulse-width-modulated (PWM) signal which does not change polarity between positive and negative during the picking of a sheet. Step b) includes controlling the pick motor by comparing an actual pick motor velocity with a desired pick motor velocity.
Several benefits and advantages are derived from one or more of the previously-described first through fourth methods of the invention. More accurate control over the pick and feed operations is achieved by having sheet position for obtaining both desired pick and feed motor velocities be determined at any one time by only one of the pick and feed systems. This avoids inaccuracies in coordinating the desired velocities of two systems when both desired velocities are dependent upon, but use different values for, sheet position due to error buildup from manufacturing tolerances and resolution limits in the components of the two systems. By having the pick system be in contact with the sheet when sheet position is determined by the pick system for obtaining desired velocities and having the feed system be in contact with the sheet when sheet position is determined by the feed system for obtaining desired velocities insures that contact with the sheet is never lost in determining sheet position for obtaining desired velocities. By having the pick motor feedback controlled wherein the actual pick motor velocity is always determined by the pick system (instead of being determined by the feed system after the happening of the predetermined event) and having the feed motor feedback controlled wherein the actual feed motor velocity is always determined by the feed system (instead of being determined by the pick system before the happening of the predetermined event) simplifies implementation of motor control since velocity depends on changes in position over time and not on actual position and therefore actual velocity determination is immune to inaccuracies in determining position.
Several benefits and advantages are derived from one or more of the previously-described fifth and sixth methods of the invention. Starting the pick operation with an input to the pick motor sufficient to prevent any teetering rotational motion of the encoder wheel will prevent any teetering signal transitions which would be falsely counted as motion along the forward direction leading to an erroneous determination of sheet position. Driving a DC pick motor with a PWM signal which does not change polarity between positive and negative during the picking of a sheet will prevent counter-rotational driving of the encoder wheel which would be falsely counted as motion along the forward direction leading to an erroneous determination of sheet position.